Display surfaces in image display devices such as liquid crystal displays (LCDs) or cathode ray tube display devices (CRTs) are required to cause no significant reflection of light applied from external light sources such as fluorescent lamps from the viewpoint of enhancing the visibility of the image.
The phenomenon that the reflectance is reduced by coating a low refractive index transparent film onto the surface of a transparent object has hitherto been known. The visibility can be improved by providing an antireflection film utilizing this phenomenon onto the display surface of an image display device. Antireflection films are classified into those having a single-layer construction comprising a low refractive index layer onto the display surface and those having a multilayer construction comprising one or a plurality of layers having a medium to high refractive index provided on the display surface from the viewpoint of improving antireflection properties and a low refractive index layer provided on the medium to high refractive index layer(s).
The antireflection film having a single layer structure has a simpler layer construction than the antireflection film having a multilayer structure and thus is superior in productivity and cost performance. On the other hand, the antireflection film having a multilayer structure can improve antireflection properties through a combination of layer constructions and can realize improved performance more easily than the antireflection film having a single-layer structure.
Methods for low refractive index layer formation are generally classified roughly into gas phase methods and coating methods. Gas phase methods include physical methods such as vacuum deposition and sputtering and chemical methods such as CVD, and coating methods include roll coating, gravure coating, slide coating, spray coating, dip coating, and screen printing.
When the low refractive index layer is formed by the gas phase method, a high-function and high-quality transparent thin film can be formed, but on the other hand, accurate atmosphere control in a high vacuum system is necessary. Further, the gas phase method involves a problem that, since a special heating device or ion generating accelerator is used, the production apparatus is complicated, and, thus, the size of the production apparatus is so large that the production cost is necessarily increased. In addition, in the gas phase method, it is difficult to form a large-area transparent thin film or to evenly form a transparent thin film on the surface of a film or the like having a complicated shape.
On the other hand, the formation of the low refractive index layer by spraying, i.e., one of the coating methods, is disadvantageous, for example, in that the utilization efficiency of the coating liquid is low and the regulation of film formation conditions is difficult. The formation of the low refractive index layer by roll coating, gravure coating, slide coating, dip coating, screen printing or the like can realize good utilization efficiency of the film forming material and thus is excellent in mass production and equipment cost, but on the other hand, in general, it involves a problem that the transparent thin film formed by the coating method is inferior to that formed by the gas phase method in function and quality.
It is known that the formation of the low refractive index layer by the coating method may be carried out by coating a coating liquid comprising a polymer containing fluorine atoms in its molecule onto a surface of a base material and drying the coating, or by coating a coating liquid containing a monomer having in its molecule a functional group(s) curable upon exposure to an ionizing radiation or heat onto a surface of a base material, drying the coating, and then applying UV, heat or the like to the dried coating to cure the coating.
In the coating film comprising a fluorine atom-containing binder, the refractive index decreases with increasing the fluorine atom content. Further, the antifouling property improves with increasing the fluorine atom content of the coating film. However, increasing the fluorine atom content of the coating film disadvantageously lowers the hardness and strength of the coating film.
Another method for lowering the refractive index known in the art is that the refractive index of the whole coating film is lowered by bringing the wavelength of air having a refractive index of 1 to not more than the wavelength of the visible light and incorporating this air into the coating film.
Japanese Patent Laid-Open No. 64601/2000 discloses a low refractive index layer having a very fine porous structure formed by forming microvoids having an average diameter of not more than 200 nm in a coating film formed of a fluorine atom-containing polymer. This technique, however, involves a problem that, when the amount of microvoids for lowering the refractive index is excessively large, the hardness and strength of the coating film are lowered.
Japanese Patent Laid-Open No. 3501/1994 discloses a low refractive index layer having a very fine porous structure formed by utilizing voids and the like formed by extraction or gas evolved, for example, by heat decomposition. As with the above technique, however, the technique disclosed in this publication involves a problem that, when the amount of these voids is excessively large, the hardness and strength of the coating film are lowered.
Further, Japanese Patent Laid-Open Nos. 167637/2001 and 225866/2002 disclose a low refractive index layer comprising hollow fine particles having cavities in the interior thereof such as hollow silica dispersed in a binder containing an inorganic component obtained from a hydrolytic polycondensate of an alkoxysilane. This low refractive index layer has the same effect as attained by the low refractive index layer containing a large number of microvoids and, at the same time, has a certain level of coating film hardness because the microvoid is protected by a hard outer shell such as silica.
This low refractive index layer is a highly hard coating film by virtue of the use of an inorganic component-containing binder, but on the other hand, the low refractive index layer suffers from a problem that, due to its poor in resistance to external impact, the mechanical strength, particularly scratch resistance, of the coating film is poor.
Further, by virtue of the aggregating effect of the hollow silica particles, the hardness of the coating film is higher than that of the hollow silica particle-free coating film, but on the other hand, since brittleness is also increased, it is difficult to realize a low-refractive index and highly mechanically strong coating film.
In an earlier application filed by the applicant of this application, there is disclosed an antireflection film having improved mechanical strength, formed by using porous fine particles, which are expected to have the effect of forming microvoids, using fine particle aggregates having voids formed by aggregation among particles, and using, as a binder component, an ionizing radiation curing-type resin composition which can easily regulate mechanical properties (such as elasticity) of the film.
Since, however, increasing the addition amount of the silica particles to some extent causes aggregation of silica fine particles which causes a lowering in mechanical strength of the coating film at a stretch, there is still a demand for an antireflection film which has a low refractive index and excellent mechanical strength.